sgsn/test: Extend tests to simulate lost GSUP requests

This patch extends test_gmm_attach to optionally resend Attach
Requests until the SGSN responds to it instead of calling
OSMO_ASSERT at a few places. The test_gmm_attach_subscr_gsup_auth
test optionally uses this feature. It is called once in either mode.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/sgsn/sgsn_test.ok b/openbsc/tests/sgsn/sgsn_test.ok
index a3c0342..c9c0165 100644
--- a/openbsc/tests/sgsn/sgsn_test.ok
+++ b/openbsc/tests/sgsn/sgsn_test.ok
@@ -12,6 +12,7 @@
 Auth policy 'remote', auth faked: Testing GMM attach
 Auth policy 'remote', triplet based auth: Testing GMM attach
 Auth policy 'remote', GSUP based auth: Testing GMM attach
+Auth policy 'remote', GSUP based auth: Testing GMM attach with retry
 Testing GMM reject
   - Attach Request (invalid MI length)
   - Attach Request (invalid MI type)